In the conventional zinc electrowinning process, calcined zinc concentrates are leached with spent (zinc depleted) electrolyte recycled from the electrowinning operation. The acidity of this return liquor, which ranges from about 100 to about 225 g/l H.sub.2 SO.sub.4) is such that some of the iron present in the zinc-bearing calcine dissolves along with the zinc. As the leach proceeds, the acid in the return liquor is gradually consumed and the pH rises to a terminal value generally between pH 4 and pH 6. This leaching operation is referred to as the "neutral leach" and the filtrate after the solids/liquids separation as the neutral leach filtrate.
Iron which initially dissolved, if present, in the trivalent oxidation state, becomes insoluble at about pH 3 and precipitates from solution. The iron precipitate forms voluminous gelatinous flocs which occlude highly deleterious insoluble impurities, such as antimony, arsenic or germanium and ensure their removal from solution. This function of iron is so important that additional iron salts are added to the solution if an insufficient amount is provided by the calcine. On the other hand, divalent (ferrous) iron is soluble under the processing conditions; consequently, it is imperative that all the dissolved and added iron be oxidized to the trivalent form prior to the solid/liquids separation otherwise the divalent iron will not be removed in the subsequent purification steps and will remain in the solution sent to the electrowinning operation. There the iron will contaminate the zinc metal product and increase energy consumption.
It is common practice to oxidize the iron present in the leach solution by the addition of an excess of a strong oxidant such as permanganate ions or manganese dioxide. The soluble excess oxidant, particularly the permanganate ion, however, consumes zinc dust during the subsequent purification step in which zinc dust is added to cement more noble impurities. The overall reaction is shown in Equation 1 below. EQU 2KMnO.sub.4 +5Zn+8H.sub.2 SO.sub.4 .fwdarw.2MnSO.sub.4 +K.sub.2 SO.sub.4 +5ZnSO.sub.4 +8H.sub.2 O Eq. 1
As will be readily appreciated from Equation 1, for every gram of permanganate ion in solution 1.374 grams of zinc dust are consumed. This amount of zinc dust is equivalent to about 1% of the zinc production in the system and represents a significant production cost.